More than 5,000 security guards in London's financial district have been instructed by police to report people taking photographs, recording footage or even making sketches near buildings, the Guardian has learned.

City of London police's previously unseen advice singles out people who may appear to be "legitimate tourists" to prevent reconnaissance by al-Qaida.

The document, obtained under the Freedom of Information Act, helps explain a number of recent cases in which photographers have been stopped and searched by police using section 44 of the Terrorism Act, after first being approached by security guards.

There has been concern over the misuse of section 44, which allows officers to stop and search anyone without need for suspicion in designated areas. It has been repeatedly used to question tourists, photographers and film-makers. Cases have also been documented where artists have been stopped from painting in the street.

The police advice to security guards states: "In this period of heightened alert, we must report possible reconnaissance to the police and develop a culture of challenging suspicious behaviour."

Under "examples of suspicious behaviour", the document lists people spotted in stationary vehicles watching buildings or who ask "detailed or unusual questions" about a location. It was warns about people seen "loitering at or near premises for long period" and advises guards to be alert to "overheard conversations that indicate suspicious intent".

Another category of suspicious behaviour is described as: "People using recording equipment, including camera phones, or seen making notes or sketches for no apparent reason". One line in the document, marked in bold, states: "The person you think is a legitimate tourist may be somebody else!"

There is no reference to the legal rights of photographers, or the need to treat members of the public cordially.

The advice is part of Project Griffin, a police initiative to ensure private security personnel function as their "eyes and ears" to combat crime and terrorism. Most police forces and several ports across the UK have co-opted the scheme. City of London police alone have held 67 training days under the initiative.

Both the Association of Chief Police Officers (Acpo) and John Yates, Britain's most senior counter-terrorism officer, have warned that police risk losing the support of the public through the inappropriate use of section 44.

Earlier this year, the European court of human rights ruled that the "arbitrary" stop and search under section 44 without suspicion was illegal.

But while many senior police officers have sought to rein-back the use of the powers by encouraging their officers to use "common sense", City of London police, which has jurisdiction of London's Square Mile, has sought to defend the action of its officers and highlight the importance of vigilance.

In December the architectural photographer, Grant Smith, was stopped while photographing the spire of Sir Christopher Wren's Christ Church. This week he was stopped again as he took images of the skyline at One Aldermanbury Square.

He said two uniformed officers detained him, one by grabbing his arms behind his back, and they refused requests to record the stop and search on his camera. On both occasions, Smith was first questioned by a security guard who asked him not to photograph a nearby building.